1. Technical Field
The present invention relates to an aftertreatment system and control strategy in which a diesel oxidation catalyst and a filter that collects particulate matter are provided in an exhaust pipe of an internal combustion engine, and more particularly a diesel engine.
2. Description of the Related Art
According to exhaust gas regulations for diesel engines, reducing particulate matter (“PM” hereafter) is equally as important as reducing NOx. A PM collecting unit using a filter known as a DPF (Diesel Particulate Filter) is known to be effective in removing PM from exhaust gas.
In an operating condition where an exhaust gas temperature is low, PM gathers in the DPF continuously, and therefore active regeneration for removing the PM through combustion must be performed on the DPF by forcibly increasing the temperature of the exhaust gas.
This active regeneration process will now be described using FIGS. 7 and 8. A diesel oxidation catalyst (DOC) is provided in an exhaust pipe on an upstream side of the DPF.
As shown in FIG. 7, fuel is injected into a combustion cylinder in a main injection, whereupon the fuel injected in the main injection is burned to generate an engine output. Following the main injection, fuel is injected in an early post-injection. As shown in FIG. 7, the early post-injection is performed at a start time t1 of the active regeneration process. The fuel injected in the early post-injection is burned in a high-temperature atmosphere in the combustion cylinder, and the diesel oxidation catalyst is heated to an activation temperature by a high temperature generated through combustion of early post-injection (DOC heating stage A).
Next, before bottom dead center (a time t2 in FIG. 8), more fuel is injected in a late post-injection. The fuel injected in the late post-injection is oxidized by the activated diesel oxidation catalyst, and the temperature of the exhaust gas is raised to or above 600° C. by heat of reaction generated at this time (DPF inlet temperature increasing stage B). By increasing the temperature of the exhaust gas to or above 600° C., the PM collected in the DPF is burned and thus removed from the DPF.
As shown in FIG. 9, however, when the diesel engine shifts from a normal operation to a low rotation, low load operation such as an idling operation during the active regeneration process of the DPF, the PM collected in the DPF may burn abnormally such that the DPF is abnormally heated. In FIG. 9, a curve X is a temperature at an inlet of a DPF device, a curve Y is an internal temperature of the DPF device, and a curve Z is a pressure of the exhaust gas flowing through the aftertreatment system and exhaust pipe.
When the diesel engine shifts from a normal operation to a low rotation, low load operation such as an idling operation during the active regeneration process of the DPF, an oxygen concentration of the exhaust gas increases and an amount of heat removed from the DPF by the exhaust gas decreases. Accordingly, the internal temperature of the DPF increases, leading to an increase in a combustion speed of the PM collected in the DPF. Therefore, as shown in FIG. 9, the internal temperature of the DPF device may increase abnormally, and as a result, the DPF may break due to heat-generated melting or cracks. Further, in a case where a catalyst is coated on the DPF, thermal deterioration may occur in the catalyst.
Patent Document 1 discloses an aftertreatment system in which a DPF device is provided in an exhaust pipe of a diesel engine. In this aftertreatment system, the temperature of the DPF is detected, and when it is determined that PM collected in the DPF may burn rapidly, control is performed during throwing out of a clutch that engages and disengages the internal combustion engine and a load thereof to switch rotation of the internal combustion engine to a post-clutch disengagement rotation speed gradually at a gentler rate than normal. It is disclosed that in so doing, an abnormal temperature increase in the DPF can be prevented while ensuring that a sufficient amount of exhaust gas flows into the DPF.
Patent Document 2 discloses an aftertreatment system in which a diesel oxidation catalyst and a DPF device are provided in an exhaust pipe of a diesel engine, including an EGR pipe for returning a part of the exhaust gas to an intake pipe, and a control prohibition unit that closes an EGR valve and prohibits reduction of an exhaust gas flow when an accumulated amount of PM collected in the DPF is detected to be equal to or greater than a predetermined amount and a temperature of the DPF is detected to be equal to or higher than a predetermined temperature while an internal combustion engine is in a no load or idling operation condition. In so doing, the exhaust gas flow is maintained and the DPF is cooled by removal of heat.
Patent Document 2 also proposes that, in conjunction with the aforesaid control, a post-injection be performed such that additional fuel injected in the post-injection reacts with oxygen in the exhaust gas, thereby reducing an amount of oxygen in the exhaust gas. Patent Document 2 discloses that in so doing, combustion of the PM is suppressed, thereby preventing an abnormal temperature increase in the DPF.    Patent Document 1: Japanese Patent Application Publication No. 2004-68804    Patent Document 2: Japanese Patent Application Publication No. 2003-27921